National Hail Climatology Switzerland

The project National Hail Climatology Switzerland („Hagelklima Schweiz“) aims at developing a consistent and spatially explicit hail climatology for Switzerland. A new generation of radar observations and reference data and modern statistical methods promise a significant improvement over the currently used hazard maps and will therefore facilitate precise risk assessments.
The lead partners in the project are the Federal Office for Meteorology and Climatology MeteoSwiss, the Federal Office for the Environment (FOEN), the Federal Office for Civil Protection (FOCP), the Federal Office for Agriculture (FOAG), the Prevention Foundation of the Association of Cantonal Building Insurers (VKG / PSFP), the Swiss Insurance Association (SVV), the Swiss Hail Insurance, and the Swiss Society of Engineers and Architects (sia). All project partners delegate stakeholders, who are closely involved in the project design through regular discussion meetings. This guarantees targeted and user-oriented product development. The University of Bern is an additional associated consulting partner, contributing world-leading scientific expertise on hail research.

Hail is one of the three costliest natural hazards in Switzerland. Every year, hail storms cause millions of Swiss Francs in damage, surpassed only by storms and flooding. Buildings, vehicles and agriculture are assets that are particularly at risk. Forecast and warnings are essential nececcisties for risk management, but also insurers and the building sector require reliable and qualitative information on current and long-term hail risk.

The localized occurrence and short duration of hailstorms
make it hard to accurately measure hail on the ground. As a consequence,
systematic, spatially comprehensive observations of hail are largely missing in
Switzerland. In the past, hail maps have been generated using damage observations
from insurance data, hail reports, and point-scale observations. These maps are
inconsistent and often based on data which are not quality-controlled, or were
calculated with now outdated methods.

The National Hail Climatology Switzerland („Hagelklima
Schweiz“)project) project aims at
developingconsistent, high-quality
climatologies of hail frequency and intensity in Switzerland, which are run and
updated operationally. Within the frame of the project, the most recent
generation of radar data is reprocessed and improved and innovative statistical
methodologies are developed and applied. Information on hail tracks, but also
products that take a closer look at individual hail events complement the
climatological maps and datasets.

A close and regular exchange with the stakeholders from
all project partners ensures that deliverables are approached in targeted and
user-oriented ways and that they will be ready to be used in practical
applications.

Two MeteoSwiss teams, one in Zurich (climatology) and one
in Locarno Monti (radar observations), closely collaborate to develop and
implement the national hail climatology. The adequate treatment of inevitable
uncertainties, such as those inherent to the estimation of recurrence intervals
of rare extreme hail events, is a particular requirement within this project.

In addition, it is ensured that all developed products
and climate information will continuously be updated and always be based on the
latest radar observations, guaranteeing long-term and high-quality operation
and provision hosted at MeteoSwiss.

Since November 2019, the project “Hagelklima Schweiz” is
one of the priority themes declared by the National Centre for Climate Services
(NCCS). More information on the products and detailes on the methods used can
be found on the web pages of the priority theme “Hail Climate Switzerland”.

Parallel projects

In parallel projects, MeteoSwiss, the Mobiliar Lab for Natural Risks at the University of Bern, the inNET Monitoring AG, and the Mobiliar Insurance Company collaborate on developing new reference data to refine and validate automatic radar hail detection technology. An entirely new kind of information is provided by the so-called “crowd-sourcing” data, i.e.hail reports collected through the MeteoSwiss mobile app (see also the MeteoSwiss Blog series on hail 2019, part 3), available in German, French, and Italian.

In addition, within the scope of the “Swiss hail observation network” project, 80 automatic hail sensors are being deployed and operated in the hail hotspots of Switzerland. The observation network will run from 2018 to 2025 and will deliver unique measurements of the size and kinetic energy of hail stones reaching the ground app (see also the MeteoSwiss Blog series on hail 2019, part4), available in German, French, and Italian.

From improved radar observations …

Because hail occurs on very small spatio-temporal scales,
it is very difficult to measure on the ground in an accurate and representative
way. Direct measurements, such as those by so-called “hailpads” or digital hail
sensors (see info box “Swiss hail observation network”) would need to be
available in very high spatial density over the entire area of Switzerland in
order to deliver useful information on hail occurrence over the country.

Observations from the five radar stations of the Swiss
radar network cover the entire area of Switzerland and thus present a unique
data basis for developing a new Swiss national hail climatology.

By using the data from weather radars and applying
specifically developed algorithms,the
probability of hail and even the expected size of hailstones can be estimated
on a 1 km by 1 km raster covering Switzerland. Such estimates ground on
empirically established relations between the altitude of the freezing level
and the altitude of specific radar echoes.

In the course of the project, the hail detection
algorithms currently implemented at MeteoSwiss will be reviewed and improved
where possible. This will provide a new generation of radar data for the
statistical estimation ofclimatological
frequencies and intensities of hail storms.

Short movie about the functionality of a weather radar.

… to new climate data for hail risk assessment

Climatologies, describing the long-term statistics of weather, inform us on where and when hailstorms of certain intensities can be expected. Such information is relevant, for example, for insurance companies calculating portfolios, but also for the building sector to adequately plan prevention measures such as building codes for roofs.

Because of incomplete measurements and short observational records of hail, existing climatological maps are spatially coarse, and the magnitude of uncertainties is largely unknown.

Improved radar data now provide the opportunity to recalculate a hail climatology for Switzerland with unprecedented spatial accuracy. In addition, exceedance probabilities and intensities of hail storms can be analyzed in high spatio-temporal resolution. To deliver robust statements and quantitative estimation of uncertainties, the project builds on internationally renowned expert knowledge and years of expertise in extreme value statistics at MeteoSwiss. The hail hazard information will be made available to the public after the project is finished.